Coupling assembly for coupling a rod to a bone anchoring element, and polyaxial bone anchoring device

ABSTRACT

A coupling assembly is provided comprising
     a receiving part ( 5, 5 ′) having a first end ( 5   a ), a second end ( 5   b ) and a central axis (C) extending through the first end ( 5   a ) and second end ( 5   b ),
       a recess ( 52 ) for receiving a rod ( 100 ), the recess having a bottom, and   an accommodation space ( 54, 54 ′) for accommodating a head ( 3 ) of an anchoring element ( 1 ), the accommodation space having an opening ( 55, 55 ′) at the second end ( 5   b ) sized so as to permit the insertion of the head ( 3 ) and a bore extending from the accommodation space to the first end ( 5   a ),   a retainer element ( 6, 6 ′) configured to be positioned at least partially in the accommodation space ( 54, 54 ′) and sized so as to at least partially encompass an inserted head ( 3 );   wherein the retainer element ( 6, 6 ′) comprises at least one spring portion ( 66   a,    66   b;    66 ′) that is compressible in an axial direction and that extends in an axial direction at least up to a position toward the first end ( 5   a ) of the receiving part that is higher than the bottom of the channel ( 52 ) for the rod.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/926,691, filed Jan. 13, 2014, the contents ofwhich are hereby incorporated by reference in their entirety, and claimspriority from European Patent Application EP 14151004.0, filed Jan. 13,2014, the contents of which are hereby incorporated by reference intheir entirety.

BACKGROUND

1. Field of the Invention

The invention relates to a coupling assembly for coupling a rod to abone anchoring element and a polyaxial bone anchoring device with such acoupling assembly. The coupling assembly comprises a receiving part witha channel for receiving the rod, an accommodation space foraccommodating a head of a bone anchoring element and a retainer elementfor retaining the head of the bone anchoring element in the receivingpart. The retainer element comprises a structure that includes a springportion that is biased in such a manner that the retainer element snapsautomatically onto a head of the bone anchoring element when the head isbeing inserted.

2. Description of Related Art

From US 2013/0150852 A1 a polyaxial bone anchor including a housing, abone screw, and a retainer for pivotably coupling the head of the bonescrew to the housing is known. The retainer is positioned into the boreof the housing and includes a plurality of alternating tabs and slotscircumferentially arranged to define a cavity for receiving the headportion of the bone screw therein. The bone anchor further includes aresilient spring means biasing the retainer towards the lower end of thehousing. The head portion of the bone screw may apply a force againstthe retainer opposing and overcoming the biasing force of the resilientspring means. The resilient spring means may be, for example, a wavewasher, a helical spring, an elastomeric member, etc. or may becircumferential or helical slots formed in the retainer.

U.S. Pat. No. 8,075,603 B2 describes a fastening system consisting of apolyaxial ball and socket joint used in conjunction with a bone screwhaving threads on one end and a spherical connector on the other endoperating as a pivot point about which a connection assembly moves in apolyaxial fashion. A substantially U-shaped connecting assembly has alower receptacle that operates as a socket for housing an upper retainerring and a lower split retainer ring. The socket is receptive to thespherical connector which is inserted through the lower split retainerring causing a momentary displacement thereof which allows for thepositioning of the spherical connector between the upper and lowerretainer rings. A resilient component positioned between the upperretainer ring and the connecting assembly permits relative predeterminedplacement and retention of the spherical connector relative to theconnector assembly.

The known polyaxial bone anchors allow for inserting the spherical headof a bone screw into the receiver by pushing it against the spring forceof a resilient member. However, there is still a need for a couplingassembly and a polyaxial bone anchor with such a coupling assembly thatis improved with regard to several aspects such as the efficiency andsafety of the coupling.

SUMMARY

It is an object of the invention to provide a coupling assembly forcoupling a rod to a bone anchoring element and a polyaxial bone anchorcomprising such a coupling assembly that allows to safely connect thebone anchoring element to the coupling assembly with low insertion forcebut high retention force and that requires only a small axial travel toinsert the bone anchoring element into the coupling assembly.

The object is solved by a coupling assembly according to claim 1 and bya polyaxial bone anchor according to claim 16. Further developments aregiven in the dependent claims.

The coupling assembly includes a receiving part with an accommodationspace for accommodating the head of a bone anchoring element and aretainer element configured to be positioned at least partially in theaccommodation space. The retainer element comprises at least one springportion that is compressible in an axial direction and extends in anaxial direction up to at least a position that is higher than a bottomof a channel for the rod. Therefore, the biased spring portion generatesa sufficient spring force that facilitates the snap-over of the retainerelement on the head of the bone anchoring element. The spring portionmay be realized by a snake spring that is small-sized in the radialextension seen from a central axis of the receiving part and that can belarge in an axial direction.

The retainer element further may have at least one horizontal slit atits bottom end that contributes to a low insertion force required forinserting the head into the receiving part. Simultaneously, a retentionforce that holds the head in the receiving part is high compared to theinsertion force. Therefore, the bone anchoring element is effectivelyprevented from being pulled-out from the lower opening. In addition,because of the small insertion path, milling under the head or stickingout of the head from bone can be avoided.

The coupling assembly may further include a pressure element forexerting pressure onto the head of a bone anchoring element to lock thebone anchoring element in a specific angular position relative to thereceiving part. The retainer element may encompass the pressure elementfrom the outer side thereof so that no additional height of thereceiving part for accommodating the retainer element and the pressureelement is necessary. Hence, a low profile implant is provided.

In addition, the receiving part is monolithic and sized such that theretainer element and the pressure element can be mounted from the topopening thereof.

The pressure element may be held in a position such that the head of thebone anchoring element is held by a frictional force exerted by thepressure element onto the head. The frictional force may be such thatpivoting of the head is still possible by applying a force to overcomethe frictional force.

The coupling assembly can be assembled in situ with a bone anchoringelement that has been already inserted into a bone or a vertebra.

With the coupling assembly a modular polyaxial bone anchoring device canbe provided that may include several bone anchoring elements differingin regard of their length of the shank, their anchoring features of theshank, such as different thread types, thread pitches, differentdiameter of the shank, and in regard of cannulated or non-cannulatedshanks.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become moreapparent from the description of various embodiments using theaccompanying drawings. In the drawings:

FIG. 1 shows a perspective exploded view of a first embodiment of a boneanchoring device.

FIG. 2 shows a perspective view of the bone anchoring device of FIG. 1in an assembled state.

FIG. 3 shows a perspective view from above of a receiving part accordingto the first embodiment.

FIG. 4 shows a perspective view from the bottom of the receiving partshown in FIG. 3.

FIG. 5 shows a top view of the receiving part shown in FIGS. 3 and 4.

FIG. 6 shows a cross-sectional view of the receiving part shown in FIGS.3 and 4 along line A-A in FIG. 5.

FIG. 7 shows a perspective view from above of a retainer elementaccording to the first embodiment.

FIG. 8 shows a perspective view from the bottom of the retainer elementof FIG. 7.

FIG. 9 shows a side view of the retainer element of FIGS. 7 and 8.

FIG. 10 shows a top view of the retainer element of FIGS. 7 to 9.

FIG. 11 shows a cross-sectional view of the retainer element of FIGS. 7to 9 along line B-B in FIG. 10.

FIG. 12 shows perspective view from above of a pressure elementaccording to the first embodiment.

FIG. 13 shows a perspective view from the bottom of the pressure elementof FIG. 12.

FIG. 14 shows a top view of the pressure element of FIGS. 12 and 13.

FIG. 15 shows a cross-sectional view of the pressure element of FIGS. 12and 13 along line D-D in FIG. 14.

FIG. 16 shows a cross-sectional view of the coupling assembly accordingto the first embodiment in an assembled state, the section taken in aplane perpendicular to the rod axis of a rod to be inserted.

FIG. 17 shows a cross-sectional view of the coupling assembly of FIG.16, rotated by 90°.

FIGS. 18 and 19 show cross-sectional views in a plane perpendicular tothe rod axis and rotated by 90°, respectively, of the coupling assemblyat first step of mounting the bone anchoring element.

FIGS. 20 and 21 show cross-sectional views perpendicular to the rod axisand rotated by 90°, respectively, of a second step of inserting the boneanchoring element into the receiving part.

FIG. 22 a shows a cross-sectional view perpendicular to the rod axis ofa third step of inserting the bone anchoring element into the couplingassembly.

FIG. 22 b shows an enlarged cross-sectional view of a detail of FIG. 22a.

FIG. 23 shows a cross-sectional view of the coupling assembly of FIG. 22a, the plane of the cross-section rotated by 90°, i.e. in a planecontaining the rod axis.

FIGS. 24 and 25 show cross-sectional views in a plane perpendicular tothe rod axis and rotated by 90°, respectively, of a fourth step ofinserting the bone anchoring element into the coupling assembly.

FIGS. 26 and 27 show a cross-sectional view in a plane perpendicular tothe rod axis and rotated by 90°, respectively, of a final step ofassembling the coupling assembly and the bone anchoring element.

FIGS. 28 and 29 show cross-sectional views of the fully assembledpolyaxial bone anchoring device with inserted rod and locking element ina plane perpendicular to the rod axis and in a plane rotated by 90°,respectively.

FIG. 30 shows a perspective exploded view of a polyaxial bone anchoringdevice according to a second embodiment.

FIG. 31 shows a cross-sectional view in a plane perpendicular to a rodaxis of an inserted rod of the assembled polyaxial bone anchoring deviceof FIG. 30.

FIG. 32 shows a perspective view from above of the receiving part of thepolyaxial bone anchoring device according to the second embodiment.

FIG. 33 shows a perspective view from the bottom of the receiving partof FIG. 32.

FIG. 34 shows a bottom view of the receiving part of FIGS. 32 and 33.

FIG. 35 shows a top view of the receiving part of FIGS. 32 to 34.

FIG. 36 shows a cross-sectional view of the receiving part of FIGS. 32to 35 along line F-F in FIG. 35.

FIG. 37 shows a perspective view of the retainer element of thepolyaxial bone anchoring device according to the second embodiment.

FIG. 38 shows a side view of the retainer element of FIG. 37.

FIG. 39 shows a top view of the retainer element of FIGS. 37 and 38.

FIG. 40 shows a perspective view from above of the pressure element ofthe polyaxial bone anchoring device according to the second embodiment.

FIG. 41 shows a perspective view from the bottom of the pressure elementof FIG. 40.

FIG. 42 shows a cross-sectional view of the pressure element of FIGS. 40and 41, the cross-section taken in a plane perpendicular to a rod axisof an inserted rod.

DETAILED DESCRIPTION

As shown in FIGS. 1 and 2, a bone anchoring device according to a firstembodiment includes a bone anchoring element 1 in the form of a bonescrew having a threaded shank 2 and a head 3. The head 3 has a sphericalsegment-shaped outer surface portion 3 a, including a greatest outerdiameter E of the sphere, and a free end with a recess 3 b forengagement with a screwing-in tool. The bone anchoring device furtherincludes a coupling assembly 4 for coupling a stabilization rod 100 tothe bone anchoring element 1. The coupling assembly 4 includes areceiving part 5, a retainer element 6 and a pressure element 7configured to be arranged in the receiving part 5. The pressure element7 serves for locking the head 3 in the receiving part 5. Pins 8 a, 8 bmay also be provided for holding the pressure element 7 in the receivingpart 5.

In addition, a locking element 9 in the form of an inner screw isprovided for securing the rod 100 in the receiving part 5 and forlocking the whole device.

Referring in particular to FIGS. 3 to 6, the receiving part 5 is amonolithic part that has a first end or top end 5 a and an oppositesecond end or bottom end 5 b and a central axis of symmetry C passingthrough the top end 5 a and the bottom end 5 b. A bore 51 is providedthat is coaxial to the central axis C. In a first region adjacent to thefirst end 5 a the receiving part 5 has a substantially U-shaped recess52 with a bottom directed towards the bottom end 5 b and two freelateral legs 52 a, 52 b extending towards the top end 5 a. On the legs52 a, 52 b, an internal thread 53 is provided that cooperates with thelocking element 9. The channel formed by the U-shaped recess 52 is sizedso as to receive the rod 100 therein for connecting at least two or aplurality of bone anchoring devices. In a region below the legs 52 a, 52b, there is an accommodation space 54 that serves for receiving the head3 of the bone anchoring element and for receiving at least partially theretainer element 6 and the pressure element 7. The accommodation space54 has a first section 54 a with a slightly larger upper inner diameterthan the inner diameter of the coaxial bore 51 wherein the first sectiontapers towards the bottom end 5 b of the receiving part. In theembodiment, the section 54 a has a hollow cone shape, however, anothershape may also be contemplated. The accommodation space furthercomprises a second portion 54 b that conically tapers towards the bottomend 5 b. The second portion 54 b forms a seat for a portion of theretainer element 6. Between the two portions 54 a, 54 b a shoulder 54 cis provided that projects inward and serves as a stop for a portion ofthe retainer element 6 as described below.

The accommodation space 54 further has an opening 55 at the bottom end 5b, the inner diameter of which is larger than the greatest outerdiameter E of the head 3 of the bone anchoring element 1 so that thehead 3 can be inserted from the bottom end 5 b. The legs 52 a, 52 b eachinclude a through-hole in the form of a transverse pin hole 56 a, 56 bthat is located substantially centrally in the legs and that serves forreceiving the pins 8 a, 8 b. The pins 8 a, 8 b have such a length, thatonce a pin is inserted into a respective pin hole 56 a, 56 b, it extendsa short distance into the bore 51 to provide a stop for securing therotational position of the pressure element 7. Furthermore, the pins 8a, 8 b also have the function of providing an abutment for the springportion of the retainer element 6. The pins 8 a, 8 b may be flush withthe outer surface of the receiving part 5 when inserted.

Referring more in detail to FIGS. 7 to 11, the retainer element 6 willbe explained. The retainer element 6 is a substantially sleeve-like partwith a first end or top end 6 a and an opposite second end or bottom end6 b. It is hollow between the top end 6 a and the bottom end 6 b. Theretainer element comprises a first portion 61 in the form of a closedring that has an outer conical surface tapering towards the bottom end 6b and an inner hollow cylindrical surface. The first portion 61 is sizedso that it fits into the lower portion of the first section 54 a of theaccommodation space 54 and abuts against the conical inner surface ofthe first section 54 a. An inner diameter of the inner cylindricalsurface of the first portion 61 is greater than the largest outerdiameter E of the head 3.

Between the first portion 61 and the bottom end 6 b, the retainerelement 6 has the form of a slit ring 62. The slit ring 62 has asubstantially conical outer shape that matches the inner shape of thesecond section 54 b of the accommodation space. An inner surface 62 a ofthe slit ring 62 has the shape of a hollow spherical segment thatmatches the spherical shape of the head 3 so that the slit ring 62provides a seat for the head 3 to provide a ball and socket jointbetween the receiving part 5 and the bone anchoring element 1. The slitring 62 is formed by a first vertical slit 62 b extending from thebottom end 6 b in a substantially vertical direction. From the verticalslit 62 b two opposite horizontal slits 62 c, 62 d extendcircumferentially around the central axis C. The horizontal slits 62 c,62 d end in widened end portions 62 e, 62 f. The portion between the endportions 62 e, 62 f forms a connecting portion 62 g that connects theslit ring 62 to the first portion 61 of the retainer element. Hence, theslit ring 62 is integrally connected to the rest of the retainer element6. The width of the vertical slit 62 b and of the horizontal slits 62 d,62 f as well as the width of the connecting portion 62 g may be selectedsuch that a desired flexibility of the slit ring 62 is obtained. Bymeans of the slits 62 b, 62 c, 62 d, the slit ring 62 is configured tobe expanded and compressed in a radial direction.

From the first portion 61 two upstanding rod supporting projections 63a, 63 b that are offset from each other by 180° extend towards the topend 6 a. The upstanding projections 62 a, 62 b have an outer and innercylindrical surface portion and a groove 64 provided at their free endthat may be substantially V-shaped and sized so as to receive rods ofdifferent diameter. The rod supporting projections 63 a, 63 b extend upto such a height, that the bottom of the V-shaped groove 64 is slightlyabove the bottom of the U-shaped recess 52 of the receiving part 5 whenthe retainer element 6 is seated in the receiving part 5. Beneath thebottom of the V-shaped groove 64 a transverse pin hole 65 a, 65 b isprovided at each of the rod supporting projections 63 a, 63 b. The pinholes 65 a, 65 b extend fully through the rod supporting projections 63a, 63 b and serve for receiving a pin for coupling the retainer element6 to the pressure element 7 as explained below.

Between the rod supporting projections 63 a, 63 b there are two springportions 66 a, 66 b, that are provided on either side of the V-shapedgroove 64 and that project in an axial direction over the rod supportingprojections 63 a, 63 b, such that the top end 6 a of the retainerelement 6 is formed by the end surface of the spring portions 66 a, 66b. The spring portions 66 a, 66 b each have an outer cylindrical surfacewith an outer diameter that is only slightly smaller than an innerdiameter of the bore 51 of the receiving part so that, as can be seenfor example in FIG. 26, the spring portions 66 a, 66 b fit into the bore61. The spring portions 66 a, 66 b further have an inner cylindricalsurface that is configured to fit around an outer cylindrical surface ofthe pressure element 7, as can also be seen in FIG. 26. The design ofthe spring portions 66 a, 66 b resembles that of a snake spring. Eachspring portion 66 a, 66 b has alternating substantially verticalsections 67 and horizontal sections 68 wherein the horizontal sections68 are preferably longer than the vertical sections 67 and wherein thevertical sections 67 and the horizontal sections 68 form a meanderingstructure, meandering around a central axis c that extends parallel tothe central axis C of the receiving part 5 when the retainer element 6is seated in the receiving part. By means of this, the retainer element6 is compressible in a vertical direction when pressure is exerted ontothe spring portions 66 a, 66 b such that it changes the height from afirst height to a second height that is smaller than the first height.An uppermost horizontal section 68 a of each of the spring portions 66a, 66 b may have a cylinder segment-shaped recess 69 for receiving a pin8 a, 8 b and has a free end. It shall be noted that the length of thevertical sections 67 and the horizontal sections 68 and other parametersof the spring portions 66 a, 66 b may be selected such that a desiredresiliency is achieved. Also, the vertical sections 67 and thehorizontal sections need to be exactly vertical or horizontal but may beinclined. The spring portions 66 a, 66 b may be easily manufactured bycutting a cylindrical tube. A height of the retainer element is suchthat when the retainer element 6 is seated in the receiving part 5 withthe slit ring 62 arranged in the second portion 54 b of theaccommodation space 54, the uppermost horizontal section 68 a of thespring portions 66 a, 66 b abuts against the pins 8 a, 8 b provided inthe receiving part 5.

Referring more in detail to FIGS. 12 to 15, the pressure element 7 is asubstantially cylindrical part with a first end that is a top end 7 aand an opposite second end that is a bottom end 7 b. Adjacent to thebottom end 7 b there is a substantially spherical segment-shaped recess71 that matches the spherical surface of the head 3 and provides a headcontacting surface portion. Adjacent to the top end 7 a a rod receivingchannel 72 is provided that has two substantially flat and parallel sidewalls 72 a, 72 b and a substantially V-shaped bottom 72 c. The size ofthe channel is such that the distance between the side walls 72 a, 72 bis only slightly larger than a diameter of a largest rod to be receivedtherein. The pressure element 7 further comprises a coaxial bore 73 thatallows to access the recess 3 b in the head of the bone anchoringelement 1 with a tool.

As can be seen in particular in FIGS. 12 and 13, at either side of theV-shaped bottom of the channel 72 an elongate hole 74 a, 74 b (only oneof which is shown in the Figures) extends in an axial direction. Theelongate holes are 74 a, 74 b configured to receive pins 10 a, 10 b (seeFIG. 17) for coupling the retainer element 6 and the pressure element 7together. The pins 10 a, 10 b can move in the elongate holes 74 a, 74 bin an axial direction limited by the upper and lower ends of theelongate holes 74 a, 74 b. At each side wall of the channel 72, aU-shaped recess 75 a, 75 b extends from the top end 7 a to a distancetherefrom. The U-shaped recesses 75 a, 75 b are open to the top end 7 aand are configured to be engaged by the pins 8 a, 8 b. The bottom of therecesses 75 a, 75 b provides a stop for the pins 8 a, 8 b, respectively.

The bone anchoring device as a whole or in parts may be made of abio-compatible material, such as a bio-compatible metal or a metalalloy, for example titanium, stainless steel, a nickel titanium alloy,for example Nitinol, or of bio-compatible plastic materials, such as,for example polyether ether ketone (PEEK) or of a bio-compatible ceramicmaterial.

To provide the coupling assembly 4, the pressure element 7 and theretainer element 6 may be pre-assembled in such a manner that, as can beseen in particular in FIGS. 16 and 17, the pressure element 7 extendsinto the retainer element 6 such that the V-shaped groove 64 of theretainer element 6 is aligned with the channel 72 of the pressureelement. The pins 10 a, 10 b are inserted into the pin holes 65 a, 65 bof the retainer element 6 and extend into the elongate holes 74 a, 74 bof the pressure element 7. The pins 10 a, 10 b are mounted in asubstantially press-fit manner in the holes 65 a, 65 b. By means ofthis, the retainer element 6 and the pressure element 7 are movablerelative to each other in a translatory manner, whereby the movement islimited by the abutment of the pins 10 a, 10 b at the upper and lowerends of the elongate holes 74 a, 74 b.

The pre-assembled retainer element 6 and the pressure element 7 areinserted from the top end 5 a through the coaxial bore 61 into thereceiving part 5, so that the slit ring 62 is seated in the secondportion 54 b of the accommodation space as can be seen in FIGS. 16 and17. As further depicted in FIGS. 16 and 17, after the pre-assembled unithas been inserted into the receiving part 5, the two pins 8 a, 8 b areinserted into the pin holes 56 a, 56 b of the receiving part until theirfront ends project into the coaxial bore 51. A first function of thepins 8 a, 8 b is to provide a stop for the spring portions 66 a, 66 b. Asecond function of the pins 8 a, 8 b is to limit the path of movement ofthe pressure element 7 in the direction towards the top end 5 a of thereceiving part 5. A third function is to secure the pressure element 7and the retainer element 6 against rotation.

As shown in particular in FIG. 17, during assembly of the retainerelement 6 with the pre-assembled pressure element 7, the pins 8 a, 8 bare located at the upper end of the U-shaped recesses 75 a, 75 b. Theretainer element 6 is pressed down by the two spring portions 66 a, 66b. The pressure element 7 rests with the upper end of the recessed 74 a,74 b on the pins 10 a, 10 b, respectively, and is still movable withrespect to the retainer element 6.

The slit ring 62 is seated in the conical second portion 54 b of theaccommodation space and the pins 8 a, 8 b hold the spring portions 66 a,66 b. When the slit ring 62 is placed into the seat 54 b, its lower edge6 b projects slightly out of the lower opening 55.

Next, as depicted in FIGS. 18 and 19, the head 3 of the bone anchoringelement 1 is inserted through the lower opening 55 into the receivingpart 5. The head 3 first enters into the lower portion of the slit ring62 that projects slightly through the lower opening 55.

The slit ring 62 is pushed out of its seat 54 b as further depicted inFIGS. 20 and 21. Simultaneously, the assembly of the retainer element 6and the pressure element 7 is moved upward towards the top end 5 a ofthe receiving part 5, whereby the pins 8 a, 8 b enter deeper into theV-shaped recesses 75 a, 75 b of the pressure element 7. During theupward movement, the spring portions 66 a, 66 b are compressed, becausetheir upper ends 68 a with the recess 69 abut against the pins 8 a, 8 b.In addition, the slit ring 62 is expanded when the head 3 further entersinto it. Because the retainer element 6 and the pressure element 7 arecoupled together via the pins 10 a, 10 b, the pressure element 7 ismoved upward.

Referring further to FIGS. 22 a to 23, a further insertion of the head 3causes a further expansion of the slit ring 62 and further compressionof the spring portions 66 a, 66 b. As soon as the counterforce exertedby the compressed spring portions 66 a, 66 b is greater than thenecessary force for expanding the slit ring 62 and for sliding the slitring 62 over the portion of the largest diameter E of the head 3, thespring force of the compressed spring portions 66 a, 66 b causes theslit ring 62 to snap over the head 3 so that its lower edge 6 b easilyslides over the region with the largest diameter E (see FIG. 22 b). Thehead 3 can be inserted only to such an extent that it abuts against thehead contacting surface portion of the spherical recess 71 of thepressure element 7. The bottom of the U-shaped recesses 75 a, 75 b ofthe pressure element serves as an abutment for the pins 8 a, 8 b so thatthe pressure element with the retainer element 6 can not escape throughthe top end 5 a.

As shown in FIGS. 24 and 25, when the head 3 is fully inserted, so thatthe pins 8 a, 8 b abut against the bottom of the U-shaped recesses 75 a,75 b, the spring portions 66 a, 66 b can fully expand, thereby movingthe pins 10 a, 10 b downward until they abut against the bottom of theelongate holes 74 a, 74 b. The slit ring 62 is positioned below theportion with the largest diameter E of the head 3. The sliding of theretainer element 6 over the head 3 is supported by the spring portions66 a, 66 b and is therefore quick and safe. When the slit ring 62 isbelow the portion with the greatest diameter E of the head 3, the head 3can no longer be pulled-out through the lower opening.

As further shown in FIGS. 26 and 27, when the receiving part 5 is pulledupward or the pressure element 7 is pressed downward, the head 3 pressesthe slit ring 62 into the seat 54 b. This condition is a pre-lockingcondition wherein the head 3 is connected to the coupling assembly suchthat is cannot be separated therefrom without applying a specialinstrument.

The inner portion of the slit ring 62 may have a slight undersize withrespect to the size of the head 3 such that the slit ring 62 encompassesthe head 3 thereby exerting a frictional force onto the head 3. Hence,in the condition shown in FIGS. 26 and 27, the head is held by frictionwithin the receiving part and therefore, the receiving part can bemaintained in a specific angular position with respect to the boneanchoring element 1 before locking of the bone anchoring elementrelative to the receiving part. Also the spring force exerted by thespring portions 66 a, 66 b may contribute to the friction hold of thehead 3 in the receiver.

When the rod is mounted in this condition and moved downward with thelocking element 9, the pressure element 7 presses against the head 3which itself presses against the slit ring 62. Final tightening of thelocking element 9 locks the whole device. The fully locked condition isshown in FIGS. 28 and 29.

In use, the bone anchoring element may be inserted into the bone or intoa vertebra prior to mounting the coupling assembly. In an alternativemanner of use, the bone anchoring element and the coupling assembly arepre-assembled and thereafter inserted into the bone. A plurality of boneanchoring devices can be connected through a stabilization rod.

It shall be noted that while two pins 8 a, 8 b and 10 a, 10 b are shown,the basic function may be achieved also with only one pin for couplingthe retainer element 6 and the receiving part 7 and only one pin forholding the receiving part 7 and for forming an abutment for thesprings.

Referring to FIGS. 30 and 31, a second embodiment of the polyaxial boneanchoring device will be described. The polyaxial bone anchoring deviceaccording to the second embodiment differs from the polyaxial boneanchoring device according to the first embodiment in the design of thecoupling assembly. All parts that are similar or identical to theprevious embodiment are marked with the same reference numerals and thedescription thereof will not be repeated.

The coupling assembly 4′ includes a receiving part 5′, a retainerelement 6′, a pressure element T and one pin 8′. Referring in additionto FIGS. 32 to 36, the receiving part 5′ comprises an asymmetricaccommodation space 54′. The accommodation space 54′ has a first portion54 a′ that extends between the coaxial bore 51 downward from the bottomof the U-shaped recess 52 and a seat portion 54 b′ that has a sphericalsegment shape with a limited length in a circumferential direction. Thesize of the spherical section is such that it matches the size of alower portion of the head 3. Hence, the spherical section 54 b′ providesa partial spherical seat for the head 3. From the spherical section 54b′, a compartment 54 d′ extends from a bottom shoulder 54 c′ up to aheight that is above the bottom of the U-shaped recess 52. Thecompartment 54 d′ enlarges the accommodation space asymmetrically withrespect to the central axis C so that at one of the legs 52 a there isan enlarged space for receiving the retainer element 6′ according to thesecond embodiment. The lower edge 54 c′ of the compartment 54 d′ servesas a support for supporting the retainer element 6′ as shown in FIG. 31.The lower opening 55′ of the receiving part 5′ has a greater diameter inthe region of the compartment 54 d′ compared to the region of thespherical section 54 b′. In addition, a transverse pin hole 56′ forreceiving the pin 8′ and a circumferential extending undercut portion57′ above the pinhole 56′ are provided in the receiving part 5′.

The retainer element 6′ comprises a main portion 61′ that has an outercylinder segment-shaped contour and an inner cylinder segment-shapedcontour that resembles a sickle in a top view, a height and a width incircumferential direction so as to encompass at least a portion of thespherical head 3. The main portion 61′ fits into the lower portion ofthe compartment 54 d′ such that it can rest on the support 54 c′. Fromthe main portion 61′ a spring portion 66′ extends upward towards the topend 6 a which is shaped similar to a snake spring however with asickle-shaped contour in a top view as shown in FIG. 39. The number andlength as well as the distance of the substantially vertical section 6Tand substantially horizontal sections 68′ that form the meanderingdesign of the spring portion 66′ are adjusted such as to achieve adesired resiliency and spring force. The retainer element 6′ isconfigured to be inserted into the compartment 54 d′. Once it isinserted into the compartment 54 d′, it extends in an oblique mannerrelative to the central axis C of the coupling assembly as depicted inFIG. 31.

The pressure element T comprises at its top end 7 a two lateral collarportions 76 a, 76 b that extend from a V-shaped groove 72 c′ beyond anouter diameter of the cylindrical body of the pressure element. Thecollar portions 76 a, 76 b have such an outer size that they can extendinto the undercut portion 57 provided above the pin hole 56′ in thereceiving part 5′. Below one of the collar portions, in the embodimentshown below the collar portion 76 b, an elongate U-shaped recess 75′ isprovided that is open towards the bottom end 7 b of the pressure element7′. The U-shaped recess 75′ serves for receiving a portion of the pin 8′therein.

Referring in particular to FIG. 31, the coupling assembly may bepre-assembled such that the retainer element 6′ is inserted from the topend 5 a of the receiving part through the coaxial bore 51 until it isseated in the compartment 54 d′ and its main portion 61′ rests on thesupport 54 c′. The height of the compartment 54 d′ is such that thespring portion 66′ may be slightly pre-tensioned when the retainerelement 6′ is placed into the compartment 54 d′. The inner sphericalsurface 61 b extends into the accommodation space 54′ and forms a secondportion of a seat for the head 3 that encompasses a portion of the head3 on an opposite side compared to the spherical seat 54 b′.

The pressure element T is also inserted from the top end 5 a in such amanner that the collar portions 76 a, 76 b are aligned with the rodreceiving channel 72. Then the pressure element is moved downward untilit reaches the undercut portion 57. In this position, it can be rotatedso that its V-shaped groove 72 c′ is aligned with the channel 52 of thereceiving part and the collar portions 76 a, 76 b still extend into theundercut portion 57. Thereafter, the pin 8′ is inserted through the pinhole 56′ until its front portion extends into the U-shaped recess 75′.The pin 8′ prevents rotation of the aligned pressure element 7′.

In use, the head 3 enters through the lower opening 55′ and moves theretainer element 6′ upward, thereby compressing the spring 66′ thatmoves against the upper wall of the oblique compartment 54 d′. Thepressure element T is also pushed upward by the head 3 until its freeend surface 7 a provided on the collar portions 76 a, 76 b abuts againstthe upper surface of the undercut 57. When the head 3 presses againstthe head-contacting surface of the spherical recess 71 of the pressureelement 7′, the spring force of the spring portion 66′ causes theretainer element 6′ to snap over the portion with the largest diameter Eof the head 3 until it rests on the support 54 c′ in the receiving part5′. In this position, the head 3 can no longer be removed from thereceiving part 5′.

Further modifications of the embodiments may be contemplated. Forexample, for the bone anchoring element, various different kinds ofanchoring elements can be used and combined with the receiving part.These anchoring elements may be, for example, screws with differentlengths, screws with different diameters, cannulated screws, screws withdifferent thread forms, nails, hooks, etc. For some anchoring elements,the head and the shank may also be separate parts that can be connectedto each other.

Other possible modifications of the receiving part may include, forexample, instead of the U-shaped recess being perpendicular to thecentral axis, a recess for the rod may be inclined, open to the side, orin the form of a closed channel. Other kinds of locking devicesincluding outer nuts, outer caps, bayonet locking devices, or others arealso possible. In particular, a two-part locking device, that includes afirst locking element that exerts pressure via the pressure member onlyonto the head and a second locking element that exerts pressure onlyonto the rod to lock the head and the rod independently, may also beused. In some embodiments, the inner surface portion of the pressuremember that contacts the head, may not necessarily bespherically-shaped. The inner surface portion may have any other shapethat is suitable to exert pressure onto the head.

Instead of the pin that extends through the pin hole at the receivingpart and engages an elongate recess provided at the pressure element,other retaining mechanisms can be used that permit to retain thepressure member in alignment with the receiving part and to inhibit thepressure member moving out through the top end of the receiving part.

The head of the bone anchoring element needs not to be rotationallysymmetric. For example, the head may have two opposite flat surfaceportions between two spherically-shaped outer surface portions so as toachieve pivoting in only one plane.

The spring portion of the retainer element may have another shape thatensures sufficient length of the spring portion in an axial directionand therewith sufficient spring force. For example, a helical spring mayalso be contemplated.

Instead of the slit ring, a plurality of vertically extending slits or acombination of substantially vertically and substantially horizontallyextending slits may be provided.

The seat for the slit ring and the outer surface of the slit ring needsnot to be conical. Any shape that provides for a safe holding of theslit ring may be contemplated, such as, for example a spherical shape.

While the present invention has been described in connection withcertain exemplary embodiments, it is to be understood that the inventionis not limited to the disclosed embodiments, but is instead intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims, and equivalents thereof.

1. A coupling assembly for coupling a rod to a bone anchoring element,the coupling assembly comprising a receiving part having a first end, asecond end and a central axis extending through the first end and secondend, a recess for receiving a rod, the recess having a bottom, and anaccommodation space for accommodating a head of an anchoring element,the accommodation space having an opening at the second end sized so asto permit the insertion of the head and a bore extending from theaccommodation space to the first end, a retainer element configured tobe positioned at least partially in the accommodation space and sized soas to at least partially encompass an inserted head; wherein theretainer element comprises at least one spring portion that iscompressible in an axial direction and that extends in an axialdirection at least up to a position toward the first end of thereceiving part that is higher than the bottom of the channel for therod.
 2. The coupling assembly of claim 1, wherein the spring portion iscomprised of sections that extend in a meandering manner along alongitudinal axis that is substantially parallel to or slightly inclinedwith respect to the central axis.
 3. The coupling assembly of claim 1,wherein an outer contour and an inner contour of the spring portion issubstantially cylinder segment-shaped.
 4. The coupling assembly of claim1, wherein the retainer element is a monolithic piece that includes theat least one spring portion.
 5. The coupling assembly of claim 1,wherein the at least one spring portion extends up to at least an axialposition corresponding to the center of an inserted rod.
 6. The couplingassembly of claim 1, further comprising a pressure element configured tobe positioned at least partially in the accommodation space and beingconfigured to exert pressure onto the head.
 7. The coupling assembly ofclaim 6, wherein the at least one spring portion of the retainer elementextends at least partially around the pressure element in acircumferential direction.
 8. The coupling assembly of claim 6, whereinthe bore in the receiving part is sized such that the pressure elementand/or the retainer element is insertable into the receiving part fromthe first end.
 9. The coupling assembly of claim 6, wherein a stop isprovided for the pressure element that prevents upward movement of thepressure element once it has been inserted into the receiving part. 10.The coupling assembly of claim 1, wherein a stop is provided for theretainer element against which the retainer element can be compressed.11. The coupling assembly of claim 6, wherein the retainer element andthe pressure element are coupled together in such a manner that they canmove relative to each other a limited distance in an axial direction.12. The coupling assembly of claim 1, wherein the retainer elementcomprises a first end and a second end and a first slit spaced apartfrom the second end that extends at least partially around the centralaxis, and a second slit that extends from the second end of the retainerelement into the first slit, and wherein the first slit extends awayfrom the second slit and is longer than the second slit.
 13. Thecoupling assembly of claim 12, wherein the retainer element comprises athird slit extending partially around the central axis so that a slitring is formed at the second end of the retainer element.
 14. Thecoupling assembly of claim 1, wherein two spring portions are providedthat are arranged on either side of the channel for receiving the rod.15. The coupling assembly of claim 1, wherein a single spring portion isprovided and wherein the spring portion extends in an oblique mannerwithin an enlarged compartment of the receiving part that provides anupper and a lower stop for the retainer element.
 16. A polyaxial boneanchoring device comprising a bone anchoring element having shank foranchoring to bone and a head, and a coupling assembly according to claim1.